1. Field of Invention
This invention relates generally to internal combustion engines and more particularly to starting an internal combustion engine.
2. Description of Related Art
In general, internal combustion engines are started by applying an external starting torque to cause the engine to run up to a speed at which sustained combustion of a fuel mixture occurs, thus causing the engine to produce a drive torque. The external starting torque is commonly provided by an electric starter motor, which is mechanically coupled to the engine during starting.
Starter motors are generally configured to turn the engine from a standstill up to a low rotational speed for starting (for example 200 rpm). The starter motor must therefore be sized to provide sufficient torque to turn the engine from a state of rest to overcome peak cylinder compression.
When an engine is stopped it is likely that the engine will come to a standstill when the momentum of the pistons and other moving elements is insufficient to overcome the cylinder compression. Under these conditions it is likely that the engine will be at a point in the engine cycle close to a compression stroke, and thus the starter motor must overcome the peak cylinder compression with little opportunity to build up momentum. Accordingly a worst case starting torque must be taken into account when selecting a starter motor.
Particularly in hybrid vehicles that employ start/stop operation, an engine may be started many times during a single trip. Furthermore, in hybrid vehicles where the starter motor also functions as a generator and/or drive motor, the requirement to overcome a worst case starting torque may result in selection of a motor that does not operate optimally as a generator and/or drive motor.